N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids - Catalytic systems for Diels-Alder reaction

Article abstract of DOI:10.2478/s11532-010-0138-4

A comparative study of the Diels-Alder reactions between cyclopentadiene (1) and dienophiles (2 a - c) in N-hexylpyridinium bis(trifluoromethylsulfonyl) imide in a temperature range of 20-45°C is reported. The reaction rate constants and activation energies were calculated. Moreover, the catalytic systems based on N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids were tested as a reaction media to perform the Diels-Alder reaction. Yb, Y, Mg, Zn triflates and chlorides (0.005 to 0.1 mmol) were used as catalysts. The recycling of catalytic system consisted of YCI3 and ionic liquid was performed. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-010-0138-4

Cent. Eur. J. Chem. • 9(1) • 2011 • 192-198
DOI: 10.2478/s11532-010-0138-4
Central European Journal of Chemistry
N-hexylpyridinium bis(trifluoromethylsulfonyl)imide
and Lewis acids – catalytic systems
for Diels-Alder reaction
Research Article
Bożena Bittner*, Ewa Janus, Eugeniusz Milchert
Institute of Organic Chemical Technology, West Pomeranian
University of Technology, Szczecin, 70-310 Szczecin, Poland
Received 17 August 2010; Accepted 9 November 2010
Abstract: A comparative study of the Diels–Alder reactions between cyclopentadiene (1) and dienophiles (2 a – c) in N-hexylpyridinium
bis(trifluoromethylsulfonyl)imide in a temperature range of 20 – 45oC is reported. The reaction rate constants and activation energies
were calculated. Moreover, the catalytic systems based on N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids were
tested as a reaction media to perform the Diels-Alder reaction. Yb, Y, Mg, Zn triflates and chlorides (0.005 to 0.1 mmol) were used as
catalysts. The recycling of catalytic system consisted of YCl3 and ionic liquid was performed.
Keywords: Ionic liquids • Lewis-acids • Diels-Alder reaction • Norbornene derivatives
© Versita Sp. z o.o.
optical properties, and to enhance the thermal stability
of polymers [19] or to polymers used for packaging
1. Introduction
The Diels-Alder reaction is an example of pericyclic
reaction, resulting in the formation of cyclic product
formed from a diene and a dienophile. This reaction
is of primary importance in organic chemistry [1] and
comprises a convenient way to form a cyclic or bicyclic
product, sometimes with a heteroatom inside of the ring.
applications. The first Diels-Alder cycloaddition between
maleate anhydride and furan [20] was performed in
water as a solvent. According to the suggestion of
Breslow and Rideout the hydrophobic effect, caused by
water, is responsible for the high reaction rate [21-23].
Moreover, the hydrogen bond, formed in situ, activates
the dienophile and causes a significant enhancement of
stereoselectivity [24,25]. However, water is not a perfect
medium to perform this reaction, mostly because of the
very low solubility of the organic reagents.
Due to an increasing interest in new solvents called
ionic liquids, the Diels-Alder reaction was also carried
out in such solvents. The first solvent used for this
purpose was ethylammonium nitrate in the cycloadditon
of cyclopentadiene and methyl-vinyl ketone [26]. Due
to the following properties of ionic liquids: very low
vapour pressure, non-flammability, non-explosiveness,
thermal stability, and their ability to dissolve substrates
and metal catalysts, these liquids are widely used in the
reaction media [27,28]. Ionic liquids such as imidazolium
[29-31], phosphonium [32,33], sulfonium [34] were used
in cycloaddition reactions.
A
search for an appropriate solvent for the
cycloaddition has been a focus of intense research.
Such a solvent should meet the following requirements:
it should be nontoxic, enhance both the reaction yield
and stereoselectivity, facilitate the separation of reaction
products, and should be fully recyclable and cheap.
The norbornene derivatives have many important
applications.They are used as monomers to produce high
performance polymeric materials. The polymerization
process can be performed by ROMP (Ring Opening
Metathesis Polymerization) [2-5], a radical polymerization
[6-8] or an additive polymerization [9-11]. The norbornene
derivatives have been widely used as electron transport
species [12], in batteries [13] and in drug deliveries
[14,15], specifically as ion [16] and gene transporters
[17]. These derivatives are also used as a template
for nanoparticle synthesis [18]. They are also used as
an additive to poly(methyl methacrylate) to improve its
Ionic liquids are also used with metal chlorides as
catalysts [35-38]. A combination of metal chloride and
* E-mail: bozena.bittner@gmail.com
192

B. Bittner, E. Janus, E. Milchert
pyridinium-based ionic liquid was investigated in the medium. However, the addition of more methyl groups to
cycloaddition between cyclopentadiene and dimethyl the pyridinium ring increases viscosity, as was observed
maleate [39]. The acidic character of this system by comparison of N-hexylpyridinium [hpy][NTf2]
resulted in higher yields and higher stereoselectivities in (η=80mPa∙s) with N-hexyl-3-methylpyridinium [hmpy]
comparison with other solvents such as water, ethanol [NTf2] (η=85mPa∙s) and N-hexyl-3,5-dimethylpyridinium
and microemulsions. The application of chlorides as [hmmpy][NTf2] (η=104mPa∙s).
catalysts requires non-aqueous conditions. Water, even
Thepyridiniumionicliquidsarealsousedinthephoto-
in low amounts, causes the deactivation of the catalyst. induced electron transfer (PET) [52], the accelerated
A new generation of catalysts resistant to water was amine-catalyzed Morita–Baylis–Hillman reaction [53], for
also developed [40]. Trifluoromethanesulfonates of the tetrahydropyranylation of alcohols [54], as the ionic
metals were used in the ionic liquids or other solvents in liquid catalysts and media for the Fischer esterification
order to increase the reaction rate and stereoselectivity [55], in the extractive desulfurization of fuel [56] and the
[41-45].
Ionic
Friedel–Crafts alkylation [57].
liquids
such
as
1-ethylpyridinium
tetrafluoroborate and 1-ethylpyridinium trifloroacetate
were investigated in the cycloadditions of isoprene
with dienophiles such as acrylonitrile, acrylic acid and
methacrylic acid [46]. The ionic liquid 1-ethylpyridinium
trifloroacetate was found to be an excellent reaction
solvent enhancing both selectivity and reaction rate.
The Diels-Alder reactions were studied in catalytic
systems based on 1, 5, 10, and 20 mol% of erbium
trifluoromethanesulfonate Er(OTf)3 immobilized in the
1-methylpyridinium trifluoromethanesulfonate [47].
The addition of the catalyst significantly enhanced
reaction rates in the cycloadditions of cyclopentadiene
and dienophiles such as acrolein and ethyl acrylate.
Moreover, it is known from the literature that catalytic
systems consisting of 1-N-butylpyridinium or 1-ethyl-
3-methyl-imidazolium chlorides used in conjuction
with AlCl3 in the Diels-Alder reaction between
cyclopentadiene and acrylates enhance both reaction
rates and stereoselectivity [48]. Also the acidic
chloroaluminate ionic liquids were found to improve the
catalytic activity of the toluene-coordinated silyl borate,
enhancing reaction rates, stereoselectivity and yields of
the Diels-Alder reactions [49].
2. Experimental Procedure
2.1. Materials
The
ionic
liquid,
N-hexylpyridinium
bis(trifluoromethylsulfonyl)imide 98% pure, with water
concentration less than 120 ppm was supplied by
Merck and was additionally dried at a temperature of
45oC under a pressure of 3 mbar for 24 hours before
use. Dimethyl maleate 96%, methyl-vinyl ketone 99%
and ethyl acrylate 99% were purchased from Aldrich.
Cyclopentadiene was obtained as a result of thermal
cracking of dicyclopentadiene ≥95% pure, purchased
from Fluka. Trifluoromethanesulfonates of ytterbium
(99.99%), yttrium (98%), magnesium (97%), zinc (98%)
and chlorides of yttrium (99.99%), ytterbium (99.9%),
magnesium (98%), and zinc (99.7%) were used as
catalysts and purchased from Aldrich. Cyclohexanone
99%, used in the GC analysis, was obtained from
Merck.
2.2. Measurements
Our investigations were carried out with
bis(trifluoromethylsulfonyl)imide as an ionic liquid having
low viscosity and high thermal stability. Therefore,
it was possible to investigate the catalyst’s activity
because the influence of an anion in the ionic liquid
will not be so essential due to its weak coordinating
properties. The same ionic liquid, N-hexylpyridinium
bis(trifluoromethylsulfonyl) imide, has already been
used as a reaction medium in the intermolecular Diels-
Alder reaction [50].
A survey of the literature concerning the influence
of viscosity reveals that the longer the alkyl chain on
the nitrogen of a cation is, the measured ionic liquid’s
viscosity correspondingly increased [51]. The viscosity
of the ionic liquid should be relatively low in order to
ensure increased contact of the reagents in the reaction
Gas chromatographic (GC) measurement were
performed on a Carlo Erba GC 8000 TOP gas
chromatograph equipped with FID detector, RXITM-17
column: 30 m × 0.53 mm × 1.5 μm film layer column
(Restek). The quantitative analyses were performed
according to the internal standard method.
The water content has a significant influence on
the reaction course because it affects the physical
properties of ionic liquids such as viscosity [58]. The
catalysts undergo deactivation in the presence of water;
therefore it is essential to ensure an anhydrous medium.
The water content of the ionic liquids used in this study
were measured using the Karl–Fischer titration. If the
results were lower than 120 ppm, then it was assumed
not to have any effect on the viscosity.
193

N-hexylpyridinium bis(trifluoromethylsulfonyl)imide
and Lewis acids – catalytic systems for Diels-Alder reaction
bis(trifluoromethylsulfonyl)imide, as ionic liquid and Y,
Yb, Mg, Zn chlorides or trifluorometanosulfonates, as the
Lewis acid catalysts for the reaction of cyclopentadiene
(1) and dienophiles (2 a – c): diemethyl maleate, methyl-
vinyl ketone, ethyl acrylate (Fig.1).
2.3. Procedure for the Diels-Alder reaction
The reactions were carried out in 4 mL vials. A mixture
of N-hexylpyridinium bis(trifluoromethylsulfonyl)imide
(1 mmol) and dienophile: dimethyl maleate, methyl-
vinyl ketone or ethyl acrylate (1 mmol), was stirred
at different temperatures (20, 25, 30, 35 and 45oC).
The reaction time was measured after the addition of
cyclopentadiene. The reaction progress was monitored
by GC analysis until the reaction went to completion. The
reaction was carried out over the time needed to obtain
a yield exceeding 90% or 24 hours, whichever came
first. When the reaction was performed in the catalytic
system based on the metal salts and hexylpyridinium
bis(trifluoromethylsulfonyl)imide, a catalyst was added
to the ionic liquid before adding the dienophile.
The values of activation energies and the reaction
rate constants, obtained without catalyst, are presented
in Table 1. The reaction rate constants referring to
methyl-vinyl ketone are the highest. Dienophile reactivity
facilitated by electron-withdrawing groups on the
substituent influences the reaction rate constants in the
cycloaddition between cyclopentadiene and methyl-vinyl
ketone. A carbonyl group adjacent to the double bond
made this reagent the most active. An ester functional
group of the second dienophile (2b) did not promote
the cycloaddition to such a degree. Dienophile with two
ester groups (2c) was even less active; therefore the
second order reaction rate constants were the smallest.
Due to the rather small reaction rate constant and high
activation energy in the Diels-Alder reaction between
cyclopentadiene and dimethyl maleate, a new catalytic
system must be developed in order to enhance the
reaction rate.
2.4. Recycling
YCl3 (0.08 mmol) was was added into a 5 mL round
bottom flask. The chloride was dissolved in 1 mL of
the ionic liquid. Dimethyl maleate (4 mmol) was added
and reaction mixture was stirred with a magnetic stirrer.
Cyclopentadiene (6 mmol) was introduced into the flask
placed in the thermostatic bath at a constant temperature
in the range of 20-45°C. The reaction products, endo and
exo isomers 2,3-bis(methoxycarbonyl)-5-norbornene,
were distilled under 1 mbar pressure at a temperature
of 85oC. After the distillation was complete, the reagents
were introduced into the reaction mixture in order to
start the successive synthesis.
Yields and stereoselectivities of cycloadditions
carried out in the ionic liquids were compared with
those performed in different catalytic systems in order
to demonstrate the advantage of using Lewis acids with
ionic liquids. The catalytic activity of each system was
presented using TON TOF values (Table 2).
Methyl-vinyl ketone is the most reactive dienophile.
Due to the presence of an electron-withdrawing group,
the double bond becomes weaker, which facilitates the
Diels-Alder reaction. For this reason, even the reaction
carried only in the ionic liquid proceeded relatively fast,
which is supported by the TON TOF values in Table 2.
The reaction was almost complete after 90 min with
stereoselectivity endo:exo 85:15 without catalyst. To
enhancetheendo:exoratio,theapplicationofthecatalytic
system as a reaction medium was useful. The selectivity
with Y and Yb chlorides was increased to 92:8 after
3. Results and Discussion
In our work we have developed new and efficient catalytic
systems based on the pyridinium ionic liquid and the
Lewis acid catalysts for the synthesis of a mixture of
the endo and exo isomers 2,3-bis(methoxycarbonyl)-
5-norbornene,
2-ethoxycarbonyl-5-norbornene.
2-acethyl-5-norbornene
The
and
examined
catalytic system consisted of N-hexylpyridinium
The Diels-Alder reaction between cyclopentadiene and various dienophiles: dimethyl maleate, methyl-vinyl ketone or ethyl acrylate in
catalytic system, based on N-hexylpyridinium bis(trifluoromethylsulfonium) imide and Y, Yb, Mg, Zn chlorides or trifluorometanosulfonates,
as Lewis acid catalysts
Figure 1.
194

B. Bittner, E. Janus, E. Milchert
Table 1. The second order reaction rate constants in the Diels-Alder reaction between cyclopentadiene and various dienophiles: dimethyl maleate,
methyl-vinyl ketone or ethyl acrylate
Activation energy
k×105
(dm3 mol-1 s-1
)
Diene
Dienophile
Temperature(oC)
(kJ mol-1
)
20
25
30
25
35
45
25
35
45
14.33
15.67
19.33
1.80
2.58
5.18
0.68
1.70
2.99
2 a
2 b
2 c
22.3
1
40.6
57.0
Table 2. TON and endo:exo ratios in the Diels-Alder reaction between cyclopentadiene (1) and dienophiles (2a-c) carried in catalytic systems
metal salt + [hpy][NTf2]
Dienophiles
Catalyst
2 a
2 b
2 c
TON
[mol mol-1
Endo:exo
ratio
TON
[mol mol-1
Endo:exo
ratio
TON
Endo:exo
ratio
]
]
[mol mol-1
]
Y(OTf)3
97
97
90:10
90:10
26
19
93:7
90:10
97
98
89:11
89:11
Yb(OTf)3
Mg(OTf)2
45
88:12
18
82:18
70
88:12
Zn(OTf)2
YCl3
91
96
97
45
96
92:8
92:8
92:8
84:14
96:4
16
40
29
4
84:16
91:9
90:10
84:16
85:15
81
96
92
12
14
90:10
93:07
90:10
75:25
89:11
YbCl3
MgCl2
ZnCl2
37
30 min. In order to achieve the highest stereoselectivity, catalysts. The influence of the catalyst concentration
a number of catalysts were examined. It was found that on yields and stereoselectivity was observed. The use
ZnCl2 was the most active catalyst. With this catalyst, of larger amounts of catalyst in the system enhanced
after the reaction proceeded for 30 min, the product the yield significantly, which caused an increase in
was obtained with 94:6 endo:exo ratio, which exceeds TOF values. Examples are presented in Table 3. In the
the initial value of 85:15 measured for the cycloaddition catalytic system YCl3 + [hpy][NTf2], the obtained TOF
carried only in hexylpyridinium bis(trifluomethylsulfonyl) was 80 [mol mol-1 h-1] for 1 mol% of the catalyst, while
imide [hpy][NTf2].
for 5 mol% of the catalyst the TOF value increased to
Moreover, it was demonstrated that the presence 180 [mol mol-1 h-1]. The same tendency was noticed
of triflates in the Diels-Alder reaction between for all chlorides. However, analysing the endo:exo
cyclopentadiene and ethyl acrylate does not guarantee selectivities demonstrate that there are no significant
high TON TOF values (Table 2). The product that was differences for YCl3 and YbCl3. It was found that for
obtained after 24 h gave a lower yield than in the reaction ZnCl2, it ranges from 85:15 (1 mol%) to 93:7 (5 mol%).
carried only in [hpy][NTf2]. The only reason for using Taking into account the obtained results, it was found that
triflates as catalysts is to increase endo:exo ratio, as the search for appropriate catalysts for the Diels-Alder
was noticed with the ketone. For example, the selectivity reaction between ethyl acrylate and cyclopentadiene
ranged from 75:25 to 90:10 with Yb(OTf)3 and when the was very difficult.
ionic liquid was a reaction medium. It was found that
For some catalysts from the triflate group, such as
endo:exo ratios obtained in the catalytic systems with scandium, bismuth, indium, copper and aluminium,
chlorides were four times higher than those in the ionic oligomerization occurs which appears to be competitive
liquid. A comparison of TOF values for triflates and with the Diels-Alder reaction. This negative influence
chlorides clearly indicated that chlorides were more of oligomerization has already been noted in previous
effective catalysts for the cycloaddition. Among the studies[34]. The catalytic systems consisting of yttrium,
chlorides, YCl3 and YbCl3 stood out when taking into ytterbium and zinc triflates also caused a side reaction
account the TON TOF values and stereoselectivities. of oligomerization in the Diels-Alder reaction between
In further investigations of the cycloaddition between dimethyl maleate and cyclopentadiene.
ethyl acrylate and cyclopentadiene, the chlorides of
The Diels-Alder reaction between cyclopentadiene
yttrium, ytterbium, magnesium, and zinc were used as and dimethyl maleate conducted in [hpy][NTf2] in
195
195

N-hexylpyridinium bis(trifluoromethylsulfonyl)imide
and Lewis acids – catalytic systems for Diels-Alder reaction
Table 3. TON, TOF and endo:exo ratios dependence on catalyst concentration in the reaction between ethyl acrylate and cyclopentadiene
Catalyst
mol%
YCl3
YbCl3
3
MgCl2
5
ZnCl2
1
3
5
1
5
1
1
5
TOF
80
87
150
31
180
18
58
75
72
28
177
19
4
69
29
16
37
69
71
18
TON
Endo:exo ratio
91:9
91:9
90:10
89:11
90:10
91:9
84:16
89:11
85:15
93:7
TOF [mol mol-1 h-1)] calculated 30 min after reaction started
TON [mol mol-1] calculated 3 h after reaction started
temperature is lower than that of bistriflimides ([hpy]
[NTf2] Tdecomp=665 K, [hpy][Cl] Tdecomp=511 K). It is known
from the literature [60] that the nature of the cation has
little influence on the decomposition temperature. The
length of the alkyl chain in the cation of these ionic
liquids influences to a small degree the decomposition
temperature e.g. N-butyl-3-methylpyridinium = [bmpy]
= 670 K, N-hexyl-3-methylpyridinium = [hmpy] = 672 K
and N-octyl-3-methylpyridinium = [ompy] = 667 K. It is
more advantageous to use [hpy][NTf2], despite the fact
that the decomposition temperature for [bmpy][NTf2] is
the highest, but first measurable losses of mass were
observed at 590 K. For [hmpy] cation, these losses
were observed at 605 K. The difference between the
decomposition temperature of hexylpyridinium cation
(Tdecomp=665 K) in [NTf2], and the cation, containing one
more substituent in the hexylmethylpyridium ring, is very
small.
The TOF values and the ratio of endo:exo isomers
in the successive syntheses with recirculation of the
catalytic system ionic liquid [hpy][NTf2] and YbCl3 in the
reaction between dimethyl maleate and cyclopentadiene
are shown in Fig. 2. Probably, a decrease in the TOF
values was caused by catalyst deactivation after
oligomerization of products remaining in the reaction
media. The time required to achieve a 90% yield
increased in every cycle. The IR and UV-Vis analysis of
the ionic liquid under distillation conditions did not reveal
any significant changes in the spectra.
Figure 2. The effect of number of recycle runs on TOF values
and endo:exo ratios
the presence of yttrium or ytterbium triflates and
chlorides results in the highest values of TON
(Table 2). For this reason, the high yields were obtained
for all catalytic systems. The use of ionic liquids without
catalyst required 24 h to obtain the product with a
yield of 90%. The reaction time was shortened to less
than 1 h in the presence of the catalyst. However, the
oligomerization was sometimes observed together with
the cycloaddition in this catalytic system. Moreover, the
obtained selectivities were not as high as in the chloride
systems.
On the other hand, yttrium and ytterbium chlorides
affected the Diels-Alder reaction positively, by enhancing
yields and selectivities similar to what was previously
observed for ethyl acrylate. For YCl3 the obtained
endo:exo selectivity was 94:6. The highest endo:exo
ratio for this catalytic system was obtained with 2 mol%
of YCl3.
4. Conclusion
In addition, the recycling studies were performed
for the most active catalytic system in the Diels-Alder
reactionbetweencyclopentadieneanddimethylmaleate.
The removal of product can be carried out by extraction
or distillation under low pressure. In our studies, the
product was separated by distillation. However, the
main drawback of this method is a gradual darkening
of distillation residue due to thermal decomposition of
ionic liquid and the polymerization of reaction product.
Other researchers observed this effect as well [59].
The decomposition temperature depends primarily
on the nature of the coordinating anion. In the case of
the ionic liquids containing halides, the decomposition
The cycloaddition of cyclopentadiene with dimethyl
maleate, methyl-vinyl ketone or ethyl acrylate proceeds
smoothly in the presence of the catalytic systems
N-hexylpyridinium
bis(trifluoromethylsulfonyl)imide
and metal chlorides or triflates. In particular, chlorides
were identified to strongly affect both the yield and
stereoselectivities of the cycloaddition. The possibility
to carry out the reaction in the catalytic systems, with
the metal salt immobilized in the ionic liquids, enables
the reuse of the reaction media several times without a
significant loss of activity.
196
196

B. Bittner, E. Janus, E. Milchert
Acknowledgment
Thestudiesareco-financedbytheEuropeanUnionwithin
the European Social Fund, “Investment in knowledge
as a driving force for development of innovation in the
region” – implemented within the framework of Subaction
8.2.2 Regional Strategies of Innovation SOP HRD 2007-
2013.
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